1. Field of the Invention
The present invention relates to a method for regulating the time constant matching, for example, in DC/DC converters.
The invention particularly, but not exclusively, relates to a method to be used in DC/DC converters, for example in voltage regulator module (VRM) applications, and the following description is made with reference to this field of application for convenience of illustration only.
The invention further relates to a corresponding regulation device for regulating and controlling the time constant matching in DC/DC converters.
2. Description of the Related Art
As it is well known, the evolution of the electric characteristics of processors for PCs, WORKSTATIONS and SERVERS obliges manufacturers to search for new solutions meeting the CPU requirements that include: high precision in the supply voltage, for instance +/−0.8% under steady state conditions and +/−3% under transient conditions.
In order to ensure an accurate supply, which respects more and more restrictive specifications, it is desirable to precisely know the value of the current flowing in the coils of the DC/DC converter used as power supply unit.
In order to ensure such a precision degree, designers are induced to use a current estimate method exploiting the parasitic resistance of each coil of the DC/DC converter.
The use of this method however introduces a drawback due to the real value offset of the components employed by the respective nominal value, jeopardizing the reading precision and thus the current estimate.
By analyzing the DC/DC converter it is observed that the parasitic resistance of each coil is not directly accessible and this so as to be able to draw the current information it is necessary to use an added electric network or reading network, placed in parallel to the coil and formed by a resistance RD and by a capacitance CD, as for example shown in FIG. 1.
By now analyzing the transfer function, reported in the formula (1.1), between the voltage Vsense developing across the capacitance CD and the current IL flowing in the coil, the presence of a term can be noted which depends on the frequency and which is also called doublet.
A variation of the doublet can alter the read current information. More precisely, an overestimate of the current which flows in the coil is performed, if the time constant of the reading network, CD RD is lower than the time constant L/RL also called coil network. Whereas, in the opposite case there is a under-estimation of the read current.
                                          V            sense                    ⁡                      (            s            )                          =                                            I              L                        ⁡                          (              s              )                                ·                      R            L                    ·                                    1              +                              s                ⁢                                  L                                      R                    L                                                                                                                        1                  +                                      s                    ⁢                                                                                  ⁢                                                                  C                        D                                            ·                                              R                        D                                                                                            ︸                            doublet                                                          (        1.1        )            
In order to have a correct current estimate it is thus necessary that the time constants of the reading network and coil network are equal, such correlation is also called “time constant matching.” This condition does not normally occur due to the variation from the nominal value of the values of the components used as above hinted at, which negatively influences the time constant matching.
Let's remember that the components can depart from their nominal value due to a statistic variability, from a dependence on the temperature or simply from an ageing thereof.
Through tests carried out by the Applicant it has been observed how the use of standard components leads to differences between the two time constants which can also reach ±30%.
Thus, against a variation of the current required by the load, such difference can cause, on the DC/DC converter regulated output voltage, overelongations or subelongations which can jeopardize the tolerance band imposed by the specifications.
The effect of the overelongations and of the subelongations on the output voltage with respect to a constant ideal value is shown in FIG. 2 and it is due to a wrong Time Constant Matching.
As far as it is known, no specific solutions exist in the literature for correcting a wrong time constant matching.
A possible solution for trying to solve and/or to limit the entity of the overelongations and subelongations effects, is the use of particularly precise components, both in the coil network and in the reading network, and an oversized output capacitance.
Such solution however implies extremely high component costs linked to the precision required and, moreover, the use of such components also implies high costs in terms of area occupied on the motherboard.
Let's now analytically analyze an equation for the calculation of the tolerance band (TOB) i.e. of a DC/DC converter tolerance band with reference to the output voltage.
                              TOB          manuf                =                                                                                                                        (                                                                                                    VID                            ·                                                          k                              VID                                                                                ︸                                                reference                                            )                                        2                                    +                                                            V                      AVP                      2                                        ·                                          (                                                                                                                                                                  k                                gm                                2                                                            ︸                                                        Current                                                    Sense                                                +                                                                                                                                            k                                ESR                                2                                                                                            n                                ph                                                                                      sense                                                    element                                                                    )                                                                      ︸                            STATIC                        +                                                                                V                    AVPdyn                    2                                    ·                                                                                                              (                                                                                                                    k                                L                                2                                                            +                                                              k                                C                                2                                                                                                                    n                              ph                                                                                )                                                ︸                                            TimeConstant                                        Matching                                                  ︸                            DYNAMIC                                                          (        1.2        )            ±TOB=TOBmanuf+Vripple+VTC  (1.3)
In particular, the equation (1.3) defines the calculation for the determination of the tolerance band, which however needs the value of the TOBmanuf defined by the equation (1.2).
As it can be observed, the equation (1.2) shows the dependence of the tolerance band on some characteristic parameters dependant on the typology of the used reading or sensing network. By introducing such parameters it is possible to understand which weight the single blocks, which compose a DC/DC converter, have in the global calculation of the TOB.
The equation (1.2) collects the static and dynamic variations relative to the DC/DC converter and, for obtaining the total contribution, they are quadratically summed.
The contribution of the time constant matching in the equation (1.2) is given by the third addend, wherein the terms KL and KC appear which are respectively the coil statistic variation and the capacitance CD one. Whereas, the term nph indicates the number of the phases of each DC/DC converter and the term VAVPdyn indicates the variation of the output voltage against a variation of the required current Idyn.
The terms Vripple and VTC, reported in the equation (1.3), indicate the deterministic variations of the output voltage.
Let's now analyze the equation (1.4) which allows to highlight a residual difference between the two time constants equal to 4%.
                                                        4              ⁢                                                          ⁢              %                        =                                                                                                                              K                        L                        2                                            +                                              K                        C                        2                                                                                    n                      ph                                                                      ⇒                                  n                  ph                                            =              7                                ,          8                ⁢                                  ⁢                  where          ⁢                      :                    ⁢                                          ⁢                      K            L                          =                              10            ⁢                                                  ⁢            %            ⁢                                                  ⁢            and            ⁢                                                  ⁢                          K              C                                =                      5            ⁢                                                  ⁢            %                                              (        1.4        )            
As it can be observed, with current and known DC/DC converters a residual difference between the two time constants equal to 4% can be reached by using an eight-phase system so as to allow a greater mediation of the unbalance of each phase.
However, systems with such a high number of phases are out of the normal trend for DC/DC converters, which require reduced costs and minimal uses of areas in the motherboard.
Moreover, it is good to consider that the above indicated result of the equation (1.4) solution has been obtained by using particularly precise components and, in a particular way, as regards the output capacitance, a capacitor with COG dielectric has been used, which has a great precision, a lower dependence on the temperature and a lower aging effect with respect to the normal capacitors. However, such types of COG capacitors have a more than double cost with respect to the common capacitors.